1. Field
The following description relates to crystalline strontium titanate powder (SrTiO3) and a method of preparing strontium titanate powder (SrTiO3).
2. Description of Related Art
With the recent miniaturization of multilayer capacitors having high capacity, the dielectric elements used in such capacitors are becoming thinner. In order to fabricate a thin, miniaturized multilayer capacitor, it is necessary to reduce the particle size of particles in the dielectric element and to suppress particle growth. A composite oxide is often used as a source material of the dielectric element. Suitable composite oxide needs to have fine particle sizes and high crystallinity.
Strontium titanate (SrTiO3) powder is often used as the composite oxide material in preparing the dielectric element. The conventional method of preparing the strontium titanate powder involves a solid-state reaction using titanium oxide (TiO2) and strontium carbonate (SrCO3). However, recently, liquid-phase methods have been used for preparing powder having high performance and high purity through uniformity in particle-size distribution and nanoscale crystallization. As the liquid-phase methods used for preparing the strontium titanate powder, a hydrothermal synthesis method, a hydrolysis method, a sol-gel method, a coprecipitation method, and the like have been used.
However, in order to prepare strontium titanate by the above-described methods, a reaction temperature of at least about 150° C. or more needs to be maintained. Further, after the reaction, a hydroxide or a hydrate may form, deteriorating the crystallinity of the powder. To improve the crystallinity of the powder, a high-temperature heat treatment is carried out. However, during the heat treatment, particles may grow large due to the heat energy introduced from the heat treatment. Thus, an additional milling process may be required in order to obtain fine-size particles. The powder that results from such a process may have surface defects and may have irregular overall shape. Further, it is difficult to control powder composition, such as the ratio between Sr and Ti, and the particle size. Furthermore, unground particles may remain; thus, during mixing or molding of the powder, dispersibility is reduced and sinterability is low. Therefore, abnormal crystal grains may easily result from the process. Further, as described above, the average particle size of the obtained powder may be several hundred nanometers or more; thus, it is impossible to obtain nanoscale particles. Currently, particles are generated through a bottom-up method; however, the produced particles need to undergo post-processing steps of a top-down method. Thus, the fabrication process is complex and inefficient.
Korean Patent No. 10-0503858 relates to a method of preparing nanoscale crystalline strontium titanate powder from a titanium tetrachloride aqueous solution prepared from inorganic acid and a strontium carbonate aqueous solution, and to a method comprising: mixing a titanium tetrachloride aqueous solution with a strontium carbonate aqueous solution; adjusting a pH to obtain a precipitate; washing the precipitate to adjust a pH again; and filtering and drying to obtain strontium titanate powder. However, according to the above-described preparation method, it is necessary to separate a precipitate and an additional drying process is needed; thus, the production process is complicated. Further, when the precipitate is separated, it may contain a by-product. Therefore, strontium titanate powder prepared by the above-described method is unstable in terms of its crystal structure.
Due to the issues pointed out above, it is desirable to develop a method of preparing strontium titanate powder that has a high crystallinity due to a precise control of the composition of the compound, is not sensitive to moisture, can be mass-produced, and formed into fine powder.